High voltage windings are used in various dynamoelectric machines, such as motors or generators. For example, high-voltage windings commonly referred to as stator windings are used in high-voltage electrical generators. A high-voltage winding, such as a stator winding, can be formed from at least one winding bar that, in turn, comprises one or more electrical conductors. The individual electrical conductors are formed of a highly conductive material, such as copper. The electrical conductors are typically individually insulated and bundled together to form the winding bar. The bundle, in turn, is surrounded by insulation, and is often referred to as a winding insulator or groundwall insulator. The groundwall insulator typically includes multiple layers of an insulation tape, such as a glass-backed mica tape. Overlaying the groundwall is an outer conductive ground electrode that surrounds the groundwall insulator. The outer conductive ground electrode can be a coating of conductive paint or a wrapped conductive tape over the groundwall insulator. The outer conductive ground electrode is connected to ground so that the voltage of the outer surface of the high-voltage winding is at ground potential.
The tape may be applied half lapped, abutted or in any other suitable manner. Generally, multiple layers of the mica tape are wrapped about the coil with sixteen or more layers generally being used for typical high voltage coils. The number of layers may be decreased depending on the power of the generator and the effectiveness of the insulator in both its abilities to insulate electrically and conduct heat. To impart better abrasion resistance and to secure a tighter insulation, a wrapping of an outer tape of a tough fibrous material, for example, glass fiber, asbestos or the like may be applied to the coil.
The insulation tape is generally impregnated with a resin to improve its overall properties. There are many methods of coating materials with epoxy resins and then curing the product. One such method is vacuum pressure impregnation (VPI). This method is used on devices such as stator conductor coils. The insulation tape is applied to the coils, then the coils are placed in a vacuum vessel and a vacuum is applied. After a period of time, resin is admitted to impregnate the coils. Pressure is applied to force the resin into the coils and minimize voids, which will affect conductivity. After this is completed, the coils are heated to cure the resin. A variation of this, global VPI (GVPI), involves the process wherein dry insulated coils are wound, and then the whole stator is vacuum pressure impregnated rather than the individual coils. Due to the relatively high density of present insulation tapes, there are commonly issues with optimally impregnating such tapes, such as incomplete impregnation. Incomplete impregnation may lead to the inability of the coil to resist voltage stresses and cause premature breakdown and failure.